Previously developed night vision systems and/or image intensifiers intensify available ambient light, such as moon light or star light, to produce an output image visible to the human eye. The image intensification process converts low level light into an electron pattern which is projected, for example, onto a phosphorous screen for conversion of the electron pattern into an image visible by an observer. Light beyond 940 nM (near-infrared) is either not detected or detected with very low sensitivity by most existing image intensification systems.
Night vision systems using Gen II (S-20, S-25) and Gen III (GaAs NEA) based photocathodes have little or no photosensitivity beyond wavelengths of 940 nM. Night sky radiation begins to increase dramatically beyond 950 nM wavelengths and existing detectors normally cannot observe this increased night sky irradiance. In addition, most existing night vision systems cannot detect or utilize the active imaging capability of near-infrared based lasers such as the Nd:YAG laser with 1.06 micrometer monochromatic radiation. Because such lasers are being used in an increasing number of modern electronic devices, especially military weapons systems, a need has arisen for image intensifiers and night vision systems capable of detecting 1.06 micrometer monochromatic radiation.
Existing image intensifiers capable of intensifying near-infrared radiation are normally reflection mode devices, field assisted devices, or a combination of the two. A reflection mode photocathode is a semiconductor photocathode where electrons are emitted from its surface due to light striking this same surface. Reflection mode photocathodes are impractical for many applications due to their size. A transmission mode photocathode is normally more compact and easier to use than a reflection mode photocathode. In a transmission mode photocathode, light strikes one surface of the photocathode and electrons are emitted from the opposite side. Typically, reflection mode devices have an active layer 5-10 microns thick while transmission mode devices have an active layer 1-2 microns thick.
Existing field assisted (or transferred electron) devices have a photocathode surface that is reverse biased. Field assisted devices often suffer from gross imaging problems due to enhanced emission and dark currents from reverse surface biasing. In addition, such devices are more costly to implement into a system as additional electronic circuitry is required.
Previously developed transmission mode, non-field assisted photocathodes capable of detecting near-infrared radiation suffer from at least three major disadvantages. First, these photocathodes have a window layer formed from aluminum-indium-arsenide. An aluminum-indium-arsenide window is difficult to form using a metal organic chemical vapor deposition (MOCVD) process. Instead, an aluminum-indium-arsenide layer is normally grown using molecular beam epitaxy (MBE). Molecular beam epitaxy is normally a much slower and more expensive process than MOCVD. Accordingly, photocathodes formed from aluminum-indium-arsenide are expensive to produce. Second, because an MBE process is normally used to form one of the layers of such photocathodes, existing Gen III production equipment cannot be used to produce the photocathodes without substantial modification. Third, aluminum-indium-arsenide window layers have an optical transmission cutoff of approximately 600 nM for the window layer. This optical transmission cutoff wavelength is undesirably high. Because such photocathodes cut off shorter wavelengths of light, the resulting image quality degrades due to a lesser degree of contrast between features of the displayed image.